{{Short description|Temporary region on the Sun}} {{Missing information|scientific measurements, duration times, and sizes|talksection=Missing information|date=July 2021}} In solar physics, an '''active region''' is a temporary feature in the Sun's atmosphere characterized by a strong and complex magnetic field. Active regions are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar flares.<ref>{{cite web |last1=Zell |first1=Holly |title=Active Regions on the Sun |url=https://www.nasa.gov/image-feature/active-regions-on-the-sun |website=NASA |access-date=18 July 2021 |date=20 April 2015}}</ref> The number and location of active regions on the solar disk at any given time is dependent on the solar cycle.<ref>{{cite journal |last1=Warren |first1=Harry P. |last2=Winebarger |first2=Amy R. |last3=Brooks |first3=David H. |title=A Systematic Survey of High-Temperature Emission in Solar Active Regions |journal=The Astrophysical Journal |date=10 November 2012 |volume=759 |issue=2 |pages=141 |doi=10.1088/0004-637X/759/2/141|arxiv=1204.3220 |bibcode=2012ApJ...759..141W |s2cid=119117669 }}</ref><ref>{{cite journal |last1=Del Zanna |first1=G. |title=The multi-thermal emission in solar active regions |journal=Astronomy & Astrophysics |date=October 2013 |volume=558 |pages=A73 |doi=10.1051/0004-6361/201321653|bibcode=2013A&A...558A..73D |doi-access=free }}</ref><ref>{{cite journal |last1=Basu |first1=Sarbani |last2=Antia |first2=H. M. |last3=Bogart |first3=Richard S. |title=Ring-Diagram Analysis of the Structure of Solar Active Regions |journal=The Astrophysical Journal |date=August 2004 |volume=610 |issue=2 |pages=1157–1168 |doi=10.1086/421843|bibcode=2004ApJ...610.1157B |doi-access=free }}</ref><ref>{{cite journal |last1=Hagino |first1=Masaoki |last2=Sakurai |first2=Takashi |title=Latitude Variation of Helicity in Solar Active Regions |journal=Publications of the Astronomical Society of Japan |date=25 October 2004 |volume=56 |issue=5 |pages=831–843 |doi=10.1093/pasj/56.5.831|doi-access=free }}</ref><ref>{{cite journal |last1=Zhang |first1=Jie |last2=Wang |first2=Yuming |last3=Liu |first3=Yang |title=Statistical Properties of Solar Active Regions Obtained from an Automatic Detection System and the Computational Biases |journal=The Astrophysical Journal |date=10 November 2010 |volume=723 |issue=2 |pages=1006–1018 |doi=10.1088/0004-637X/723/2/1006|bibcode=2010ApJ...723.1006Z |s2cid=122852367 |doi-access=free }}</ref>
==Region numbers== Newly observed active regions on the solar disk are assigned 4-digit region numbers by the Space Weather Prediction Center (SWPC) on the day following the initial observation. The region number assigned to a particular active region is one added to the previously assigned number. For example, the first observation of active region 8090, or AR8090, was followed by AR8091.
According to the SWPC, a number is assigned to a region if it meets at least one of the following criteria:<ref>{{cite thesis |type=PhD |last=Pietrow |first=A.G.M. |date=2022 |title=Physical properties of chromospheric features: Plage, peacock jets, and calibrating it all. |publisher=Stockholm University| url=https://www.diva-portal.org/smash/record.jsf?aq2=%5B%5B%5D%5D&c=10&af=%5B%5D&searchType=LIST_LATEST&sortOrder2=title_sort_asc&query=&language=en&pid=diva2%3A1651858&aq=%5B%5B%5D%5D&sf=all&aqe=%5B%5D&sortOrder=author_sort_asc&onlyFullText=false&noOfRows=50&dswid=5451 | doi=10.13140/RG.2.2.36047.76968}}</ref> # It contains a sunspot group of class C or larger based on the Modified Zurich Class sunspot classification system. # It contains a sunspot group of class A or B confirmed by at least two observers, preferably with observations more than one hour apart. # It has produced a solar flare with an X-ray burst.{{Clarification|date=June 2022}} # It contains plage with a white-light brightness of at least 2.5 (on a linear scale 1-5, 5=flare) and has an extent of at least five heliographic degrees. # It contains plage that is bright near the west limb and is suspected of growing.
The region numbers reached 10,000 in July 2002. However, the SWPC continued using 4-digits, with the inclusion of leading zeros.<ref name="SRS">{{cite web |title=Solar Region Summary {{!}} NOAA / NWS Space Weather Prediction Center |url=https://www.swpc.noaa.gov/products/solar-region-summary |website=www.swpc.noaa.gov |access-date=4 November 2021}}</ref><ref name="jaeggli16" />
==Magnetic field== right|thumb|A highly simplified diagram of the magnetic field of an active region illustrating its bipolar nature. ===Mount Wilson magnetic classification=== The Mount Wilson magnetic classification system, also known as the Hale magnetic classification system, is a method of classifying the magnetic field of active regions. It was first introduced in 1919 by George Ellery Hale and coworkers working at the Mount Wilson Observatory.<ref>{{cite journal |last1=Hale |first1=George E. |last2=Ellerman |first2=Ferdinand |last3=Nicholson |first3=S. B. |last4=Joy |first4=A. H. |title=The Magnetic Polarity of Sun-Spots |journal=The Astrophysical Journal |date=April 1919 |volume=49 |pages=153 |doi=10.1086/142452 |bibcode=1919ApJ....49..153H |url=https://articles.adsabs.harvard.edu/full/1919ApJ....49..153H |access-date=29 December 2021}}</ref> It originally included only the α, β, and γ magnetic classifications, but it was later modified by H. Künzel in 1965 to include the δ qualifier.<ref>{{cite journal |last1=Künzel |first1=H. |title=Zur Klassifikation von Sonnenfleckengruppen |journal=Astronomische Nachrichten |date=December 1965 |volume=288 |page=177 |bibcode=1965AN....288..177K |url=https://articles.adsabs.harvard.edu/full/1965AN....288..177K |access-date=29 December 2021}}</ref><ref name="jaeggli16" /> {| class="wikitable" |- ! Classification !! Description<ref>{{cite book |title=Space Environmental Observations, Solar Optical Observing Techniques, Manual AFWAMAN 15-1 |date=2013 |publisher=Air Force Weather Agency |url=https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/afrl_publications/afwaman15-1_space-environmental-observations.pdf |access-date=28 December 2021}}</ref><ref name="jaeggli16">{{cite journal |last1=Jaeggli |first1=S. A. |last2=Norton |first2=A. A. |title=THE MAGNETIC CLASSIFICATION OF SOLAR ACTIVE REGIONS 1992–2015 |journal=The Astrophysical Journal |date=16 March 2016 |volume=820 |issue=1 |pages=L11 |doi=10.3847/2041-8205/820/1/L11|arxiv=1603.02552 |bibcode=2016ApJ...820L..11J |s2cid=15138687 |doi-access=free }}</ref><ref>{{cite web |title=The magnetic classification of sunspots |url=https://www.spaceweatherlive.com/en/help/the-magnetic-classification-of-sunspots.html |website=SpaceWeatherLive |publisher=Parsec vzw |access-date=29 December 2021}}</ref> |- | align="center" | α || An active region containing a single sunspot or group of sunspots all having the same magnetic polarity. An opposite polarity counterpart is still present, but is weak or not concentrated enough to form sunspots. |- | align="center" | β || An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity. A simple neutral line between the two polarities is also present. |- | align="center" | γ || An active region with sunspots having completely intermixed magnetic polarity. |- <!-- Some sources exclude the dashes between the Greek letters. For example, β-γ is sometimes written as βγ. It is included here for readability. --> | align="center" | β-γ || An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity (hence β) but no well-defined neutral line dividing the opposite polarities (hence γ). |- | align="center" | δ || A qualifier to the other classes indicating the presence of opposite polarity umbrae within a single penumbra separated by at most 2° in heliographic distance. |- | align="center" | β-δ || An active region with a β magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ). |- | align="center" | β-γ-δ || An active region with a β-γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ). |- | align="center" | γ-δ || An active region with a γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ). |}
==Sunspots== {{main|Sunspot}} right|thumb|An active region seen in visible light showing a group of sunspots. thumb|The evolution of a group of sunspots in time. The strong magnetic flux found in active regions is often strong enough to inhibit convection. Without convection transporting energy from the Sun's interior to the photosphere, surface temperature decreases along with the intensity of emitted black body radiation. These areas of cooler plasma are known as sunspots, and often appear in groups.<ref>{{cite web |title=SECEF Sunspot Resource |url=https://image.gsfc.nasa.gov/poetry/workbook/sunspot.html |website=image.gsfc.nasa.gov |access-date=2021-08-24 |archive-date=2021-11-22 |archive-url=https://web.archive.org/web/20211122225451/https://image.gsfc.nasa.gov/poetry/workbook/sunspot.html |url-status=dead }}</ref> However, not all active regions possess sunspots.<ref name="SRS" />
==Magnetic flux emergence== Active regions form through the process of magnetic flux emergence, during which magnetic fields generated by the solar dynamo emerge from the solar interior.<ref>{{cite journal |last1=van Driel-Gesztelyi |first1=Lidia |last2=Green |first2=Lucie May |title=Evolution of Active Regions |journal=Living Reviews in Solar Physics |date=December 2015 |volume=12 |issue=1 |article-number=1 |doi=10.1007/lrsp-2015-1 |s2cid=118831968 |doi-access=free |bibcode=2015LRSP...12....1V }}</ref><ref>{{cite journal |last1=Cheung |first1=Mark C. M. |last2=Isobe |first2=Hiroaki |title=Flux Emergence (Theory) |journal=Living Reviews in Solar Physics |date=2014 |volume=11 |issue=3 |doi=10.12942/lrsp-2014-3 |s2cid=122762353 |doi-access=free |bibcode=2014LRSP...11....3C }}</ref><ref name="aschwanden19">{{cite book |last1=Aschwanden |first1=Markus J. |title=New Millennium Solar Physics |series=Astrophysics and Space Science Library |date=2019 |volume=458 |location=Cham, Switzerland |doi=10.1007/978-3-030-13956-8 |isbn=978-3-030-13956-8 |s2cid=181739975 |url=https://doi.org/10.1007/978-3-030-13956-8}}</ref>{{rp|118}}
==Upflow region== Analysis of Doppler velocity maps from the Hinode/EIS instrument, which observe the solar corona in specific spectral lines like FeXII, shows that at the edges of active regions on the Sun, there are always areas where plasma (hot, ionized gas) flows upward from the corona.<ref name=":0">{{Cite journal |last1=Sakao |first1=Taro |last2=Kano |first2=Ryouhei |last3=Narukage |first3=Noriyuki |last4=Kotoku |first4=Jun'ichi |last5=Bando |first5=Takamasa |last6=DeLuca |first6=Edward E. |last7=Lundquist |first7=Loraine L. |last8=Tsuneta |first8=Saku |last9=Harra |first9=Louise K. |last10=Katsukawa |first10=Yukio |last11=Kubo |first11=Masahito |last12=Hara |first12=Hirohisa |last13=Matsuzaki |first13=Keiichi |last14=Shimojo |first14=Masumi |last15=Bookbinder |first15=Jay A. |date=2007-12-07 |title=Continuous Plasma Outflows from the Edge of a Solar Active Region as a Possible Source of Solar Wind |url=https://www.science.org/doi/10.1126/science.1147292 |journal=Science |language=en |volume=318 |issue=5856 |pages=1585–1588 |doi=10.1126/science.1147292 |pmid=18063788 |bibcode=2007Sci...318.1585S |issn=0036-8075}}</ref> These upward flows appear as blue-shifts in the Doppler velocity images. These regions of upflow are located where the Sun’s magnetic field lines are open—meaning they extend outward into space—and this connection seems to help drive the upflows.<ref name=":0" />
The upflow regions could be the sources of the slow solar wind,<ref name=":0" /><ref>{{Cite journal |last1=Harra |first1=L. K. |last2=Sakao |first2=T. |last3=Mandrini |first3=C. H. |last4=Hara |first4=H. |last5=Imada |first5=S. |last6=Young |first6=P. R. |last7=van Driel-Gesztelyi |first7=L. |last8=Baker |first8=D. |date=2008-04-01 |title=Outflows at the Edges of Active Regions: Contribution to Solar Wind Formation? |url=https://iopscience.iop.org/article/10.1086/587485 |journal=The Astrophysical Journal |language=en |volume=676 |issue=2 |pages=L147–L150 |doi=10.1086/587485 |bibcode=2008ApJ...676L.147H |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Kojima |first1=M. |last2=Fujiki |first2=K. |last3=Ohmi |first3=T. |last4=Tokumaru |first4=M. |last5=Yokobe |first5=A. |last6=Hakamada |first6=K. |date=August 1999 |title=Low-speed solar wind from the vicinity of solar active regions |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JA900177 |journal=Journal of Geophysical Research: Space Physics |language=en |volume=104 |issue=A8 |pages=16993–17003 |doi=10.1029/1999JA900177 |bibcode=1999JGR...10416993K |issn=0148-0227}}</ref> a stream of charged particles that flows constantly from the Sun into space. The upflow areas tend to form early in the development of an active region and persist throughout its lifetime.<ref>{{Cite journal |last1=Brooks |first1=David H. |last2=Harra |first2=Louise |last3=Bale |first3=Stuart D. |last4=Barczynski |first4=Krzysztof |last5=Mandrini |first5=Cristina |last6=Polito |first6=Vanessa |last7=Warren |first7=Harry P. |date=2021-08-01 |title=The Formation and Lifetime of Outflows in a Solar Active Region |journal=The Astrophysical Journal |volume=917 |issue=1 |pages=25 |doi=10.3847/1538-4357/ac0917 |arxiv=2106.03318 |bibcode=2021ApJ...917...25B |doi-access=free |issn=0004-637X}}</ref>
However, the exact reasons why these upflows occur are still not fully understood. Several hypothesis are being explored, such as waves traveling through the corona<ref>{{Cite journal |last1=Tian |first1=Hui |last2=McIntosh |first2=Scott W. |last3=De Pontieu |first3=Bart |date=2011-02-01 |title=The Spectroscopic Signature of Quasi-Periodic Upflows in Active Region Timeseries |url=https://iopscience.iop.org/article/10.1088/2041-8205/727/2/L37 |journal=The Astrophysical Journal |volume=727 |issue=2 |pages=L37 |doi=10.1088/2041-8205/727/2/L37 |arxiv=1012.5112 |bibcode=2011ApJ...727L..37T |issn=2041-8205}}</ref> or magnetic reconnection—where magnetic field lines break and reconnect—in the active region’s core with open magnetic field lines.<ref>{{Cite journal |last1=Del Zanna |first1=G. |last2=Aulanier |first2=G. |last3=Klein |first3=K.-L. |last4=Török |first4=T. |date=February 2011 |title=A single picture for solar coronal outflows and radio noise storms |url=http://www.aanda.org/10.1051/0004-6361/201015231 |journal=Astronomy & Astrophysics |volume=526 |pages=A137 |doi=10.1051/0004-6361/201015231 |bibcode=2011A&A...526A.137D |issn=0004-6361}}</ref> It’s also possible that multiple processes work together to create these upward flows at the same time.<ref>{{Cite journal |last1=Barczynski |first1=Krzysztof |last2=Harra |first2=Louise |last3=Kleint |first3=Lucia |last4=Panos |first4=Brandon |last5=Brooks |first5=David H. |date=July 2021 |title=Comparison of active region upflow and core properties using simultaneous spectroscopic observations from IRIS and Hinode |url=https://www.aanda.org/10.1051/0004-6361/202140387 |journal=Astronomy & Astrophysics |volume=651 |pages=A112 |doi=10.1051/0004-6361/202140387 |arxiv=2104.10234 |bibcode=2021A&A...651A.112B |issn=0004-6361}}</ref>
==See also== *List of solar cycles *List of solar storms *Hyder flare *Magnetic cloud *Orbiting Solar Observatory *Phoebus group, international scientists aiming at detecting solar g modes *Solar and Heliospheric Observatory
==References== {{Reflist}}
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Category:Solar phenomena Category:Solar cycles